Selection Matrix: Step 4

Step 4. Watershed Factors

The design of urban STPs is fundamentally influenced by the nature of the downstream water body that will be receiving the stormwater discharge. Consequently, the designer needs to be cognizant of the goals in the resource the site drains to. This section includes selection criteria and design guidelines for the eight resource categories included in the Watershed Templates. These include:

Sensitive Streams
Impacted Streams
Non-Supporting Streams
Restorable Streams
Aquifers
Urban Lakes
Reservoirs
Estuaries/Shellfish Beds

Many jurisdictions may have specific requirements related to a specific resource, such as a particular reservoir. Others may have identified specific protection areas that include special requirements. The watershed factors presented here are generic, and based on the watershed templates.

STP Selection Matrix 4-1. Watershed Factors - Streams

STP
GROUP SENSITIVE
STREAM IMPACTED
STREAM NON-SUPPORTING STREAM RESTORABLE
STREAM

Ponds Require channel protection.

Restrict in-stream practices.

In cold water streams, minimize permanent pool area, and encourage shading.
Require channel protection. Emphasize flood control when local flooding is a concern.

Provide long detention times for bacteria control.
Require channel protection.

Where possible, integrate design with watershed retrofit priorities.

Wetlands Require channel protection.

Restrict in-stream practices.

Restrict use in cold-water streams.
Require channel protection. Emphasize flood control when local flooding is a concern.

Provide long detention times for bacteria control.
Require channel protection.

Where possible, integrate design with watershed retrofit priorities.

Design in-stream wetland practices to support habitat restoration goals.

Infiltration Strongly encourage use for groudwater recharge.

Combine with a detention facility to provide channel protection.
Combine with a detention facility to provide channel protection. Combine with a detention facility to provide flood control where appropriate.

Avoid direct infiltration of hotspot runoff.
Combine with a detention facility to provide channel protection.

Filtering Systems
Combine with a detention facility to provide channel protection. Combine with a detention facility to provide channel protection. Combine with a detention facility to provide flood control where appropriate.

Use as pretreatment prior to an infilration practice for hotspot runoff.
Combine with a detention facility to provide channel protection.

Open

Channels
Combine with a detention facility to provide channel protection. Combine with a detention facility to provide channel protection. Combine with a detention facility to provide flood control where appropriate.

Often restricted due to space limitations.
Combine with a detention facility to provide channel protection.

Often restricted due to space limitations.

Note: For all stream systems, removal of specific pollutants may also be a goal, particularly when a stream does not meet water quality standards, is part of a TMDL watershed, or drains to a waterbody that has specific pollutant reduction targets.

STP Selection Matrix 4-2. Watershed Factors - Other Aquatic Resources
STP GROUP	AQUIFER	URBAN LAKE	RESERVOIR	ESTUARY/ SHELLFISH BEDS
Ponds	May require liner if HSG A soils are present. Pretreat hotspots. Provide a separation distance to water table.	Encourage the use of a large permanent pool to improve phosphorous removal.	Encourage the use of a large permanent pool to improve phosphorous removal. Promote long detention times to encourage bacteria removal. Require channel protection.*	Encourage long detention times to promote bacteria removal. Provides high nitrogen removal.
Wetlands
Infiltration	Provide 100' horizontal separation distance from wells and 4' vertical distance from the water table. No hotspot runoff, unless pretreated by another practice, such as a filtering system. Require pretreatment of all runoff except rooftop.	OK. Provides high phosphorous removal.	Provide a separation distance from bedrock and water table Pretreat runoff prior to infiltration practices.	OK, but provide a separation distance to seasonally high groundwater.
Filtering Systems	Excellent pretreatment for infiltration or open channel practices.	OK, but designs with a submerged filter may result in phosphorous release.	Excellent pretreatment for infiltration or open channel practices. Moderate to high coliform removal	Moderate to high coliform removal Designs with a submerged filter bed appear to have very high nitrogen removal
Open Channels	OK, but hotspot runoff must be adequately pretreated	OK. Moderate P removal.	Poor coliform removal for wet swales.	Poor coliform removal for grass wet swales.
* Although channel protection is critical to prevent erosion of upstream channels draining to urban lakes and reservoirs, both flood control and channel protection requirements may be waived for sites that drain directly to reservoirs or lakes, or larger order streams.

STP Selection Matrix 4-1. Watershed Factors - Streams
STP GROUP	SENSITIVE STREAM	IMPACTED STREAM	NON-SUPPORTING STREAM	RESTORABLE STREAM
Ponds	Require channel protection. Restrict in-stream practices. In cold water streams, minimize permanent pool area, and encourage shading.	Require channel protection.	Emphasize flood control when local flooding is a concern. Provide long detention times for bacteria control.	Require channel protection. Where possible, integrate design with watershed retrofit priorities.
Wetlands	Require channel protection. Restrict in-stream practices. Restrict use in cold-water streams.	Require channel protection.	Emphasize flood control when local flooding is a concern. Provide long detention times for bacteria control.	Require channel protection. Where possible, integrate design with watershed retrofit priorities. Design in-stream wetland practices to support habitat restoration goals.
Infiltration	Strongly encourage use for groudwater recharge. Combine with a detention facility to provide channel protection.	Combine with a detention facility to provide channel protection.	Combine with a detention facility to provide flood control where appropriate. Avoid direct infiltration of hotspot runoff.	Combine with a detention facility to provide channel protection.
Filtering Systems	Combine with a detention facility to provide channel protection.	Combine with a detention facility to provide channel protection.	Combine with a detention facility to provide flood control where appropriate. Use as pretreatment prior to an infilration practice for hotspot runoff.	Combine with a detention facility to provide channel protection.
Open Channels	Combine with a detention facility to provide channel protection.	Combine with a detention facility to provide channel protection.	Combine with a detention facility to provide flood control where appropriate. Often restricted due to space limitations.	Combine with a detention facility to provide channel protection. Often restricted due to space limitations.
Note: For all stream systems, removal of specific pollutants may also be a goal, particularly when a stream does not meet water quality standards, is part of a TMDL watershed, or drains to a waterbody that has specific pollutant reduction targets.